1.3. Macromolecular configuration

The concept of configuration includes a certain spatial arrangement of atoms of macromolecules, which does not change during thermal motion. The transition from one configuration to another is impossible without a break chemical bonds.

Distinguish: 1) link configuration, 2) short-range order - the configuration of connecting links, 3) long-range order - the configuration of large sections (for example, blocks and their alternation, or the length and distribution of branches), 5) the configuration of an elongated chain as a whole.

Link configuration. Examples are the cis and trans configurations of diene polymers

1,4-cis-polyisoprene 1,4-trans-polyisoprene (natural rubber) (gutta-percha) Another example would be l,d-isomerism. For example,

for polymers with ~CH2 –CHR~ units, where R is any radical, the formation of two isomers is possible: l is left-handed, and d is right-handed

Link attachment configuration(short order). The links in the chain can be connected in a head-to-tail and head-to-head fashion:

is head-to-tail attachment, and head-to-head attachment requires overcoming large activation barriers.

For copolymers, the types of structural isomers increase compared to homopolymers. For example, for copolymers of butadiene and styrene, it is possible:

1. sequential alternation of links -A-B-A-B-A-B-,

2. combination of links in the form of dyads and triads–AA–BBB–AA–BBB– ,

3. statistical combination of links–AA–B–AA–BBB–A–B– . Far configuration order spreads on

tens and hundreds of atoms in the main chain. For example, large sequences of blocks in block copolymers or large sequences of units with the same stereoregularity (for example, polymers with isotactic, atactic and syndiotactic structures).

Isotactic Atactic Syndiotactic

Overall circuit configuration is determined by the mutual arrangement of large sequences of links (with a long-range order). For example, for branched macromolecules, various types of configurations are shown in Fig. 4.

Rice. 4. Configurations of macromolecules

1.4. Conformation of macromolecules

A conformation is a variable distribution in space of atoms or groups of atoms that form a macromolecule. The transition from one conformation to another can occur due to rotation, rotation or oscillation of links around single bonds under the action of thermal motion or external forces and is not accompanied by the breaking of chemical bonds.

Polymers can take various conformations:

Statistical tangle is a folded conformation. It is formed when the intensity of the internal thermal motion prevails over the external influence. Characteristic of linear polymers [PE, PP, PB, PIB and ladder polymers (polyphenylenesiloxane).

Helix - is formed in polymers due to H-bonds (for example, in protein molecules and nucleic acids).

A globule is a very compact particle close to spherical in shape. It is characteristic of polymers with strong intramolecular interaction (for example, in PTFE).

Rod or string found in alkyl polyisocyanates.

Fold conformation. It is characteristic of polymers in a crystalline state (for example, in PE).

Crankshaft Conformation realized in poly-n-benzamide.

Fig.5. Conformations of macromolecules

1.5. Flexibility of macromolecules

Flexibility is one of the most important characteristics of polymers, which determines the highly elastic, relaxation, and thermomechanical properties of polymers, as well as the properties of their solutions. Flexibility characterizes the ability of macromolecules to change their shape under the influence of thermal motion of links or external mechanical influences. Flexibility is due to the internal rotation of links or parts of macromolecules relative to each other. Consider the phenomenon of internal rotation in molecules on the example of the simplest organic compound- ethane molecules.

In the ethane molecule (CH3 -CH3) carbon atoms are bonded to hydrogen atoms and to each other by covalent (σ-bonds), and the angle between the directions of σ-bonds (valence angle) is 1090 28/. This causes a tetrahedral arrangement of substituents (hydrogen atoms) in space in the ethane molecule. Due to thermal motion in the ethane molecule, one CH3 group rotates relative to the other around axes C-C. In this case, the spatial arrangement of atoms and the potential energy of the molecule are continuously changing. Graphically, various extreme arrangements of atoms in a molecule can be represented as projections of the molecule onto a horizontal plane (Fig. 6). Let us assume that in position a the potential energy of the molecule is U1, and in position b it is U2, while U1 ≠ U2, i.e. the positions of the molecule are energetically unequal. Position b, in which the H atoms are located one below the other, is energetically unfavorable, since repulsive forces appear between the H atoms, which tend to transfer the atoms to the energetically favorable position a. If accept

U1 =0, then U2 =max.

Rice. 6. Projection formulas extreme arrangements of H atoms in space in the ethane molecule.

Rice. 7. Dependence of the potential energy of the molecule on the angle of rotation of the methyl group.

When one CH3 group is rotated relative to another by 600, the molecule goes from position a to b, and then after 600 again to position a, and so on. The change in the values ​​of the potential energy of the ethane molecule from the angle of rotation φ is shown in Fig.7. Molecules with lesser symmetry (for example, the dichloroethane molecule) have a more complex dependence U=f(φ).

Potential (U 0 ) or activation barrier rotation

ion is the energy required for the transition of the molecule from the position of the minimum to the position of the maximum potential energy. For ethane, U0 is small (U0 = 11.7 kJ/mol) and at

at normal temperature, CH3 groups rotate around C-C connections at high speed (1010 rpm).

If the molecule has an energy reserve less than U0, then there is no rotation and only oscillation of the atoms occurs relative to the position of the minimum energy - this is limited or

slow rotation.

In polymers, due to intra- and intermolecular interactions, the dependence U=f(φ) has a complex shape.

If one position of the chain link is characterized by potential energy U1, and the other - by U2, then the energy of transition from one position to another is equal to the difference ∆U= U1 - U2. The difference between the transition energies ∆U from one equilibrium position of a macromolecule unit to another characterizes thermodynamic flexibility. It determines the ability of the chain to bend under the influence of thermal motion.

Another characteristic of flexibility is the speed at which links move from one position to another. The rate of conformational transformations depends on the ratio of U0 and energy external influences. The more U0 , the slower the turns of the links and the less flexibility. The flexibility of macromolecules, determined by the value of U0, is called kinetic flexible

Factors that determine the flexibility of macromolecules

These factors include: the U0 value, polymer MM, density of the spatial network, size of substituents, and temperature.

Potential rotation barrier (U 0 ). The value of U0 depends on intra- and intermolecular interactions. Let us consider the factors affecting U0 and chain flexibility in carbon-chain polymers.

Carbochain polymers

In carbon chain polymers, the least polar are saturated hydrocarbons. Their intra- and intermolecular interactions are small, and the values ​​of U0 and ∆U are also small, therefore, polymers have high kinetic and thermodynamic flexibility. Examples: PE, PP, PIB.

The values ​​of U0 are especially low for polymers, in the chain of which there is a double bond next to the single bond.

–CH2 –CH=CH–CH2 – Polybutadiene

lar groups leads to intra- and intermolecular interactions. In this case, the degree of polarity significantly affects

With the introduction of polar groups, three cases are possible in terms of their effect on flexibility:

1. Polar groups are closely spaced and strong interactions are possible between them. The transition of such polymers from one spatial position to another requires overcoming large U0, so the chains of such polymers are the least flexible.

2. Polar groups are rarely located in the chain and there is no interaction between them. The values ​​of U0 and ∆U are small and the polymers have high kinetic and thermodynamic flexibility.

-CF 2 -CF 2 -

Example: Polychloroprene

3. Polar groups are arranged so that electric fields mutually offset. In this case, the total dipole moment of the macromolecule is equal to zero. Therefore, the values ​​of U0 and ∆U are low, and polymers have high kinetic and thermodynamic flexibility.

Example: PTFE

Heterochain polymers

In heterochain polymers, rotation is possible around C–O, C–N, Si–O, and C–C bonds. The values ​​of U0 for these bonds are small and the chains have sufficient kinetic flexibility. Examples: polyesters, polyamides, polyurethanes, silicone rubbers.

However, the flexibility of heterochain polymers can be limited by intermolecular interactions due to the formation of H-bonds (for example, in cellulose, polyamides). Cellulose is one of the rigid chain polymers. It contains a large number of polar groups (–OH) and therefore intra- and intermolecular interactions and high values ​​of U0 and low flexibility are characteristic of cellulose.

Molecular weight of the polymer. An increase in the molecular weight of the polymer increases chain folding and, therefore, long macromolecules

have greater kinetic flexibility compared to short macromolecules. As the MM increases, the number of conformations that a macromolecule can adopt increases and the flexibility of the chains increases.

Spatial mesh density. The more chemical bonds between macromolecules, the less chain flexibility, i.e. as the density of the spatial grid increases, the flexibility decreases. An example is the decrease in chain flexibility with an increase in the number of crosslinks in the resol series.< резитол<резит.

Effect of size and number of substituents. An increase in the number of polar and large substituents reduces the mobility of the macromolecule units and reduces the kinetic flexibility. An example is the decrease in the flexibility of butadiene-styrene copolymer macromolecules with an increase in the content of bulky phenyl substituents in the chain.

If there are two substituents at one carbon atom in the main chain of the polymer (for example, OCH3 and CH3 in PMMA units), then the macromolecule becomes kinetically rigid.

Temperature. As the temperature rises, the kinetic energy of the macromolecule increases. As long as the value of the kinetic energy is less than U0, the chains perform torsional vibrations. When the kinetic energy of the macromolecule becomes equal to or exceeds U0, the links begin to rotate. With an increase in temperature, the value of U0 changes little, while the speed of rotation of the links increases and the kinetic flexibility increases.

Control questions

1 General information about polymers, concepts, definitions.

2 Define and give examples of organic, non-

organic and organoelement polymers.

2 Classification of homochain polymers, examples.

3 Classification of heterochain polymers, examples.

4 Thermodynamic and kinetic flexibility of macromolecules. What factors affect the flexibility of macromolecules?

5 What is the configuration of macromolecules and what types of configurations of macromolecules are possible? Examples.

6 What is the conformation of macromolecules and what types of conformations of macromolecules are possible? Examples.

7 What parameters characterize the molecular weight, molecular weight distribution and polydispersity of polymers?

8 Molecular characteristics of oligomers.

9 Fractionation of polymers and construction of molecular curves cular mass distribution.

It differs from the same polymer in the highly elastic state by the mobility of the elements of the structures of macromolecules, i.e. relaxation times: for macromolecules, segments and supramolecular formations in the glassy state, they are very large and often exceed the testing or operation time of polymers. The latter is confirmed by the fact that the value of the glass transition temperature depends on the exposure time of the polymer sample in the process of physical or mechanical impact.

Cellulose triacetate crystallization occurs in different ways depending on the conditions. In most cases, the folding of polymeric macromolecules occurs irregularly. The most pronounced crystalline regions are weakly associated with the rest of the polymer mass These pronounced crystalline regions are, as it were, immersed in the amorphous region of cellulose triacetate Non-pronounced crystalline regions are more strongly associated with the polymer mass They cannot be separated from the rest of the mass without destruction

At the same time, the rate of formation of a crystalline phase from solutions with low supersaturation is very low. Spontaneous formation of nuclei of a new (crystalline) phase requires a fluctuation combination of a group of segments of several polymeric macromolecules, and not only a combination in a strict geometric order, but also such a quantitative combination that the critical value of the nucleus is exceeded. In short, all the regularities of nucleation characteristic of low-molecular-weight systems are preserved here, with the only complicating difference being that, due to the low mobility of macromolecules, the probability of the appearance of a crystallization center is significantly reduced and a very long time or a significant supersaturation of the solution is required to separate the crystalline phase, which increases the probability aosity of fluctuation formation of nuclei. Another limitation of crystallization can be the achievement of such polymer concentrations at which the viscosity of the system becomes very high, the mobility of macromolecules sharply decreases (it practically disappears during glass transition), and crystallization is impossible. ."These extreme cases should be considered in more detail, in particular, when analyzing phase transformations in jelly.

The planar zigzag conformation in a polyethylene macromolecule can be easily achieved due to the fact that hydrogen atoms are small in size: their van der Waals radius is 0.12 nm (1.2 A). When replacing hydrogen atoms with other atoms or groups, for example, chlorine atoms [radius 0.18 nm (1.8 A) | or fluorine [radius 0.15 nm (1.5 A)], in most cases the chain can no longer maintain a planar conformation, since large atoms cause significant stresses in the macromolecule. Therefore, most polymeric macromolecules have a helical conformation. In this case, the identity period may include one or more. spiral turns. For example, in a polytetrafluoroethylene macromolecule, which has the shape of a slightly twisted helix, at a temperature below 20 ° C, the identity period [equal to 1.68 nm (16.8 A)] includes six turns of the helix, in which there are thirteen CF2 units. In the temperature range of 20–30 °C, the chain unwinds slightly, so that there are fifteen links per period of identity. The shape of the polytetrafluoroethylene macromolecule is close to cylindrical. At temperatures above 30 ° C, the structure becomes partially disordered; the chains, without violating the mutual arrangement, oscillate or rotate in concert around their axes.

The planar zigzag conformation in a polyethylene macromolecule can be easily achieved due to the fact that hydrogen atoms are small in size: their van der Waals radius is 0.12 nm (1.2 A). When hydrogen atoms are replaced by other atoms or groups, e.g. chlorine [radius 0.18 nm (1.8 A)] or fluorine [radius 0.15 nm (1.5 A)], in most cases the chain can no longer maintain a flat conformation, since large atoms cause significant stresses in the macromolecule. Therefore, most polymeric macromolecules have a helical conformation. In this case, the identity period may include one or more. spiral turns. For example, in a polytetrafluoroethylene macromolecule, which has the shape of a slightly twisted spiral, at a temperature below 20 ° C, the identity period [equal to 1.68 nm (16.8 A)] includes six turns of the helix, on which thirteen CF2 units are located. In the temperature range of 20–30 °C, the chain unwinds slightly, so that there are fifteen links per period of identity. The shape of the polytetrafluoroethylene macromolecule is close to cylindrical. At temperatures above 30 ° C, the structure becomes partially disordered; the chains, without violating the mutual arrangement, oscillate or rotate in concert around their axes.

Two types of dispersion are possible at a higher crystallite level. The first is the distribution of one component in another at the microlevel in the form of separate crystallites located in the areas of the matrix with a significant density defect; the second - the macrodistribution of one of the components as a dispersed phase, and the second type of distribution is more likely at significant concentrations of the dispersed component and in the case of poor premixing. However, in both cases one should expect the appearance of one more type of distribution of polymer macromolecules, which is characteristic of the transition region. It should be noted that when the dispersed component is distributed at the level of crystallites, it is likely that the phase boundaries will differ little from those existing in a pure polymer, which are due to the presence of amorphous and crystalline phases.

There are many experimental data confirming the presence of a transition layer at the polymer interface. So, in a mixture of rubbers, you can determine the energy of cohesion. If the mixture were single-phase, then the cohesion energy would change with the composition of the mixture along a curve lying above or below the additive one in the same way as the heat of vaporization of a mixture of low molecular weight liquids changes. In a two-phase mixture, the interaction of polymers is limited only by the interface, and in the absence of a transition layer, the interaction intensity should be low. In this case, one can think that the cohesive energy changes additively with the composition. The deviation from the additivity of the cohesion energy in a mixture of polymers indicates the presence of a transition layer in which polymer macromolecules interact. According to , the energy of cohesion in a mixture of rubbers

Plasticization of polymers is usually considered as a technological method for increasing the elastic and plastic properties of a material, i.e., reducing its fragility as a result of the introduction of specially selected low molecular weight substances - plasticizers. In this case, as is known, the transition points of the polymer from one

The word macromolecule itself is derived from the Greek word literally makros and literally translates as a large molecule. The term was first published in the works of the Nobel laureate Herman Straudinger in 1922. There are also substitute words in the literature - a polymer molecule, a polymer molecule, a macromolecular substance or a megamolecule. In the scientific interpretation, a macromolecule is a molecule with a large mass, consisting of repeatedly repeating heterogeneous or identical groups of atoms (units) connected by chemical bonds into a single chain. The number of atoms in one macromolecule reaches several million. It is considered to be a macromolecule of a substance with a molecular weight of more than 500-1000 a.m.u. Examples of macromolecules are polymers, polysaccharides, proteins, DNA and RNA.

The most important property of a macromolecule is the ability to show flexibility (change shape) under the influence of thermal energy and external mechanical action. However, the configuration of a macromolecule reflects its structure and changes only when the bond is broken at the chemical level.

Classification of macromolecules

According to the molecular weight of the macromolecule:

• low molecular weight (up to 500 amu)
• high molecular weight (from 5000 amu)

By origin of the macromolecule:

• natural (proteins, rubbers, DNA)
• synthetic (obtained during the synthesis from low molecular weight substances, for example, polyurethanes, polyolefins, polyamides, polyesters, etc.)
• artificial (obtained by processing natural polymers, such as cellulose)

By chemical composition:

• organic (the chain is formed by carbon atoms)
• organoelement (the chain contains carbon atoms)
• inorganic (there are no carbon atoms in the chain, as a rule, the chain forms oxides)

By structure:

• linear (stretched in a line chain)
• branched (chain with side branches)
• reticulated (three-dimensional cross-linked networks of macromolecules)

By structure:

• crystalline (with a stable ordered three-dimensional structure)
• amorphous (not structured)

Processing method:

• thermoplastic (when heated, they acquire viscous properties, and when cooled, they again turn into a solid body, for example, polyethylene or polypropylene)
• thermosetting (when heated, the structure is destroyed without transition to a viscous state, for example, polyurethane or epoxy resins)

Conformation of macromolecules

Conformation is a characteristic of the geometric arrangement of atoms of a macromolecule in space (ordering), which depends on the bond angles and bond lengths, and the packing of macromolecule chains, which, in turn, depends on the forces of intermolecular interactions. So one macromolecule can have a certain number of conformations, i.e. spatial structures. This is due to the fact that in long sections, under the action of thermal motion, the orientations of the bonds change. As a result, a long molecular chain takes on the statistical form of a coil (like tangled threads). The densest conformation is called a globule. The formation of such a conformation is accompanied by forces of intermolecular attraction.

Macromolecule of polyurethane foam (polyurethane)

The macromolecule of polyurethane foam (PPU) is a heterochain polymer, consisting mainly of urethane groups, but also containing functional groups of polyethers and polyesters, amide, urea and even aromatic groups. The ratio and presence of these groups in the composition of the macromolecule determines the set of physicochemical properties of the final PU foam product. So the elements of polyester in the chain of the macromolecule give PPU elasticity, and urethane and aromatic inclusions give rigidity. Aromatic groups cause an increase in physical and mechanical properties and resistance to elevated temperatures.

Classification of polymers according to the chemical structure of the main chain and the macromolecule as a whole. Intermolecular interaction in polymers. Concepts of energy density of cohesion and solubility parameter.

Structure of macromolecules includes their chemical structure and length, length and molecular weight distribution, shape and spatial arrangement of links. According to the chemical structure of the main chain, they are distinguished homochain (with a chain of carbon atoms - carbon chain ) And heterochain polymers, and according to the chemical structure of macromolecules in general - polymers:

· organic - the chain consists of carbon, oxygen, nitrogen and sulfur atoms;

· organoelement - the chain consists of silicon, phosphorus and other atoms to which carbon atoms or groups are attached, or vice versa;

· inorganic - there are no carbon atoms or carbon chains with multiple (double or triple) bonds without side groups.

Most common organic carbon chains polymers, including their various derivatives (halogen-containing, esters, alcohols, acids, etc.), the name of which is formed by the name of the monomer with the prefix "poly". Polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, polytrifluorochloroethylene, polyvinyl alcohol, polyvinyl acetate, polyacrylamide, polyacrylonitrile, polymethyl methacrylate, and others belong to the limiting aliphatic carbon chain polymers. Polybutadiene, polyisoprene and polychloroprene are unsaturated, polyethylenephenylene is an example of fatty aromatic polymers, and polyphenylene is an example of aromatic polymers. Number inorganic homochain polymers are limited - carbochain carbine (~C≡C-C≡C~) and cumulene (=C=C=C=), as well as polyser (~S-S-S~), polysilane (~SiH 2 -SiH 2 ~), polygermane (~GeH 2 -GeH 2 ~), etc. More common organoelement homochain polymers from organic chains (carbochain) with organoelement side groups or from inorganic chains with organic radicals: polyvinylalkylsilanes, polyorganosilanes, boron-containing polymers. Organic heterochains polymers are divided into classes depending on the nature of the functional groups in the backbone. They can be aliphatic or aromatic, depending on the structure of the hydrocarbon groups between the functional groups (Table 1.1).

Table 1.1.

Heterochain polymers of various classes:

Functional group	Polymer
class name	Representatives
Oxygen content
simple ethereal	Polyethers	Polymethylene oxide (~CH 2 -O~)
		Polyethylene oxide (~CH 2 -CH 2 -O~)
Ester	Polyesters	Polyethylene terephthalate ([-CH 2 -CH 2 -O-OC-Ar-CO-O-] n)
		Polyarylates ([-OC-R-COO-R`-O-] n)
		Polycarbonates ([-O-Ar-CH 2 -Ar-O-CO-O-Ar-CH 2 -Ar-] n)
Nitrogen content
Acetal	Acetals	Cellulose (C 6 H 1 0 O 5) n
Amidnaya	Polyamides (-CO-NH-)	Polyhexamethylene adipamide
Imidnaya	Polyimides	Polypyromellitimide
Urea	Polyurea	Polynonamethylene urea
Urethane	Polyurethanes (–HN-CO-O)	~(CH 2) 4 -O-CO-NH-(CH 2) 2 ~
S e r c o n t e n s
Thioether	polysulfides	Polyethylene sulfide (~CH 2 -CH 2 -S~)
Sulfonic	Polysulfones	Poly- n,n`-oxydiphenylsulfone

Inorganic heterochains the polymers are polyborazole, polysilicic acid, polyphosphonitrile chloride. Organoelement heterochain polymers include a large group of the most sought-after compounds from inorganic chains with organic side groups. These include silicon-containing polymers whose chains consist of alternating silicon and oxygen atoms ( polyorganosiloxanes ) or nitrogen ( polyorganosilazane ). Polymers with a third heteroatom in the main chain - a metal are called polymetallorganosiloxanes (polyaluminoorganosiloxanes, polyboroorganosiloxanes and polytitanorganosiloxanes). There are also polymers with organo-inorganic chains of carbon, silicon, oxygen atoms (polycarbosiloxanes, polycarbosilanes, polycarboranes), which may contain aliphatic or aromatic units. All atoms in the units of the considered polymers are connected chemical covalent bonds . There are also coordination (chelate, intracomplex) heterochain polymers, in which the units are connected by donor-acceptor interaction with a metal ion, forming coordination link (side valence) and ionic bond (main valence). Chemical and metallic bonds with a length of 0.1-0.2 nm significantly exceed the energy of physical bonds and even hydrogen bond (length 0.24-0.32 nm), which occupies an intermediate position between physical and chemical bonds. The polarity of the bonds also depends on the chemical structure and composition of the links, which is quantitatively estimated by the value of the dipole moment μ O, equal to the product of the charge and the distance between the charges (Table 1.3), as well as the level of intermolecular interaction in the polymer. Depending on the polarity of the bonds, the polymer can be polar And non-polar . The dipole moment of all organic carbon chain aliphatic (nonpolar) polymers is close to zero. Depending on the structure of macromolecules, dispersion, orientational, and induction bonds can appear between them. Dispersion bonds are due to the appearance of instantaneous dipoles in atoms during the rotation of electrons around nuclei. Polar macromolecules are characterized by orientation (dipole-dipole) bonds. In the field of dipoles of polar macromolecules, nonpolar macromolecules can also be polarized. Between permanent and induced dipoles arise induction connections.

Intermolecular interaction determines the ability of the polymer to dissolve in low molecular weight liquids, behavior at low temperatures, elastic and other properties. Its level is measured solubility parameter – the ratio of the product of the polymer density to the sum of the attraction constants of individual groups of atoms in the compound link to the molecular weight of the link. For this, they also use cohesive energy density (kJ/mol), which is equivalent to the work of removing interacting macromolecules or groups of atoms from each other over infinitely large distances. At glass transition temperature T s the energy of intermolecular interaction becomes higher than the energy of thermal motion, and the polymer passes into solid vitrified state . Polymers with T With above room is called plastics , and below room temperature and the solubility parameter 14-19 ( M . j/m 3 ) 1/2 – elastomers (rubbers).

Molecular weight of polymers and methods for its determination. Molecular weight distribution and shape of macromolecules. Classification of polymers according to the number and arrangement of constituent units.

Molecular mass(MM) - an important characteristic of the structure of polymers, which determines the level of mechanical properties and belonging to a certain group: oligomers (thermoplastics) - 10 3 -10 4, crystalline thermoplastics - 10 4 -5 . 10 4 , amorphous thermoplastics - 5 . 10 4 -2 . 10 5 , rubbers - 10 5 -10 6 . The lower the MM of polymers, the lower the viscosity of their melts and the easier they are molded. Mechanical properties are determined more by the degree of curing (oligomers) and crystallinity (polyamides, polyesters) or the transition to a glassy state. Rubbers, which are difficult to mold, have the highest MM, but products made from them have high elasticity. Since the same degree of polymerization is not obtained at high molecular weight, the macromolecules differ in size. Polydispersity (polymolecularity) - one of the basic concepts in the physical chemistry of polymers, and the type molecular weight distribution (MWD) is an important indicator that affects the physico-mechanical properties of polymers no less than MM.

Since MM is a statistical average, different methods for determining it give different values. WITH average number methods are based on determining the number of macromolecules in dilute polymer solutions, for example, by measuring their osmotic pressure, and medium-sized - on the determination of the mass of macromolecules, for example, by measuring light scattering. Average number MM ( M n ) is obtained by simply dividing the mass of a polymer sample by the number of macromolecules in it, and average mass MM: M w =M 1 w 1 +M 2 w 2 +…+M i w i , Where w 1 , w 2 , w i – mass fractions of fractions; M1 , M2 , M i – mass average MM fractions. medium viscosity MM approaching the mass average MM is determined from the viscosity of dilute solutions. The polymer is called monodisperse , if it consists of one fraction with macromolecular sizes very close to each other, and for it the ratio M w/M n =1.02-1.05. In other cases, the mass average MM is greater than the number average MM, and their ratio ( M w/M n =2.0-5.0) is a measure of the polydispersity of the polymer. The more M w/M n , the wider the MMR. On the polymer MWD curve, the value M n falls to the maximum, i.e. per fraction, the proportion of which in the composition of the polymer is the largest, and M w shifted to the right along the x-axis.

The large sizes of polymer macromolecules determined one more feature of their structure. They can be linear or branched (with side branches from the main chain or star shape). At close MM values, they become isomers . The properties of polymers consisting of linear and branched macromolecules differ greatly. branching - an undesirable indicator of the structure of macromolecules, which reduces their regularity and hinders the crystallization of the polymer. The connection of macromolecules by chemical bonds leads to the formation mesh structures , further changing the properties of polymers. In accordance with such differences in the structure of macromolecules (Fig. 1.1), polymers are also called linear , branched And reticulated (stitched ).

In the latter case, the concept of "macromolecule" loses its meaning, since the entire cross-linked polymer sample becomes one giant molecule. Therefore, in cross-linked polymers, the average value of the MM of the chain segment between the chemical bonds (network nodes) connecting the macromolecules is determined.

copolymers contain links of two or more different monomers in the main chain (for example, styrene-butadiene rubber) and have a more complex structure than homopolymers consisting of units of one monomer. A copolymer with a random combination of units of monomers in a macromolecule is called statistical , with their correct alternation - alternating , and with a large length of sections (blocks) of links of one monomer - block copolymer . If the blocks of one of the monomers are attached to the main chain of the macromolecule, composed of units of another monomer, in the form of large side branches, then the copolymer is called vaccinated . The structure of a copolymer is characterized by the chemical composition and length of the blocks or grafted chains and the number of blocks or grafts in the macromolecule. Units of the same or different monomers can be combined regularly (end of one - start of another) or irregularly (the end of one is the end of the other, the beginning of the other is the beginning of the third link, etc.), and the substituents in the side groups can have a regular or irregular spatial arrangement. The structure of a macromolecule is also determined by its configuration and conformation.

Configuration of macromolecules and stereoisomers. Conformation and flexibility of macromolecules. Flexible and rigid chain polymers and the shape of their macromolecules.

Macromolecule configuration- this is a certain spatial arrangement of its atoms, which does not change during thermal motion, as a result of which its different types are stable isomers. Cis isomers characterized by the location of different substituents on opposite sides of the double bond in each repeating unit, and trans isomers - the presence of different substituents on one side of the double bond. Examples of such isomers are NK and gutta-percha, natural polyisoprenes identical in chemical structure. Gutta-percha is a plastic with a crystalline structure, melting at 50-70 ° C, and NK is an elastomer in the temperature range from +100 O C to -72 O C, since their macromolecules have different identity periods . IN cis-polyisoprene (NA) unidirectional methyl groups meet through one compound unit, which is equal to 0.82 nm, and in his trance-isomer (gutta-percha) - after 0.48 nm:

cis- 1,4-polyisoprene (NK)

trance-1.4-polyisoprene

From macromolecules optical polymers with an asymmetric carbon atom by special methods of synthesis are obtained stereoregular isomers - isotactic (substituents - on one side of the plane of the macromolecule) and syndiotactic (deputies - on opposite sides):

They differ in properties from atactic polymers with an irregular arrangement of substituents. The mutual repulsion of the substituents leads to their displacement relative to each other in space, and therefore the plane of symmetry is bent in the form of a spiral. Spiral structure is also characteristic of biologically active polymers (for example, the DNA double helix). The structure of macromolecules of stereoisomers is a carrier of information about the methods of their synthesis, and in proteins, double helixes of DNA carry enormous information about their biological heredity.

Conformation of a macromolecule- this is the spatial arrangement of atoms or groups of atoms, which can change under the influence of thermal motion without destroying the chemical bonds between them. The large length of the macromolecule, with the possibility of rotation of its parts around fixed chemical bonds, causes rotational isomerism , which is expressed in the appearance of various conformations. The closer the hydrogen atoms are to each other ( cis-position), the greater their repulsion and, accordingly, the potential energy of the macromolecule. The interaction is enhanced by polar substituents, such as chlorine atoms. IN trance-isomers, the potential energy of the macromolecule is less, the arrangement of atoms is more favorable than in cis-isomers. Energy rotation barrier parts of a macromolecule, which makes it inhibited , consisting of a series of fluctuations, help to overcome thermal energy fluctuations . The totality of vibrations and movements around simple bonds leads to to curvature macromolecules in space, which can go in different directions and change in time. In other words, the macromolecule has flexibility - the ability to change its conformation as a result of thermal movement or the action of external forces. With a large number of atoms, the chain can not only be bent, but even curl up in very loose macromolecular coil , the size of which can be characterized root-mean-square distance between its ends and calculate mathematically, knowing the number of component links in it. Due to the chain structure of macromolecules, the movement of one atom or grouping will lead to the movement of others, resulting in a movement similar to the movement of a caterpillar or worm, which is called repational (fig.1.2). A segment of a chain that moves as a whole in an elementary act of motion is called chain segment . Thermodynamic flexibility characterizes the ability of the chain to change its conformation under the action of thermal motion and can be estimated by the stiffness parameter, the length of the thermodynamic segment, or the Flory flexibility parameter. The lower these indicators, the higher the probability of a macromolecule transition from one conformation to another (Table 1.4). Stiffness parameter evaluated by the ratio of root-mean-square distances between the ends of real and free-jointed chains in dilute polymer solutions. Length of thermodynamic segment A (Kuhn's segment) characterizes such a sequence of links, in which each link behaves independently of the others, and is also related to the root-mean-square distance between the ends of the chain. It is equal to the hydrodynamic length of a macromolecule for extremely rigid chains and the length of a repeating link for extremely flexible chains. Polymers of the diene series and with ~Si-O~ or ~C-O~ bonds in the main chain are characterized by greater flexibility compared to polymers of the vinyl series, since they, due to a decrease in exchange interactions between CH 2 -groups 100 times lower energy of rotational isomers. The nature of substituents has little effect on the flexibility of macromolecules. Flory Flexibility Parameter f O shows the content of flexible bonds in a macromolecule and serves as a criterion for flexibility, according to which polymers are divided into flexible chain (f O>0,63; A<10nm) And rigid chain (f O<0,63; A>35nm). The latter do not occur in the conformation of a macromolecular coil and have an elongated shape of macromolecules - an elastic string (polyalkyl isocyanate, A = 100), crankshaft (poly- P-benzamide, A =210) or spirals (biopolymers, A =240).Kinetic Flexibility macromolecule reflects the rate of its transition in a force field from one conformation to another and is determined by the value kinetic segment , i.e. that part of the macromolecule that responds to external influences as a whole. Unlike the thermodynamic segment, it is determined by the temperature and the speed of the external influence. With an increase in temperature, the kinetic energy and flexibility of the macromolecule increase, and the size of the kinetic segment decreases. Under conditions where the time of action of the force is longer than the time of transition from one conformation to another, the kinetic flexibility is high, and the kinetic segment is close in size to the thermodynamic segment. Under rapid deformation, the kinetic segment is close to the hydrodynamic length of the macromolecule, and even a thermodynamically flexible chain behaves as rigid. The kinetic flexibility of an isolated macromolecule is determined from the viscoelastic properties of highly dilute solutions with their subsequent extrapolation to zero concentration. Macromolecules of a flexible chain amorphous polymer have ball-shaped both in isolated form and in bulk. At the same time, the structure of the polymer is not similar to the structure of "molecular felt", in which the macromolecules are randomly entangled, as previously thought. The idea of ​​ordered regions in amorphous polymers was put forward in 1948 by Alfrey.

polymer macromolecule conformation

The form of an isolated macromolecule depends not only on the set of its configurational isomers and their location in the chain, but it is also determined by the ability of macromolecules to conformational isomerism. The latter is determined by the ability of atoms and groups of atoms of the chain to rotate around single bonds. On fig. Figure 7 shows how a 180 0 C rotation around one C-C bond in an iso-triad of a vinyl polymer leads to a change in the conformation (shape) of this triad.

Rice. Implementation of a 180 0 rotation around the C-C bond in the isotactic triad.

The conformation of a macromolecule is the spatial arrangement of atoms and groups of atoms, which is determined by the set and sequence of configurational isomers and their relative mutual arrangement in the chain, due to thermal motion or external influences on the macromolecule.

An isolated macromolecule chain in the process of thermal motion can take on a large number of different conformations, therefore the dimensions of the chain are characterized by the average distance between its ends (in this case, the root-mean-square value of the distance - is usually used). They also use the concept of the root mean square value of the radius of gyration of the chain -. The value is the average square of the distance (r i) of all elements of the mass of the chain from its center of inertia

But first, let's define the value - contour chain length- L, which is understood as the size of a hypothetical, extremely elongated chain:

Rice.

In this case, the rotation does not change the configuration of CHX atoms in the triad, since it is not accompanied by the breaking of chemical bonds. The driving force for the rotation of atoms around single bonds is their thermal motion. Under the action of thermal motion, macromolecules, due to the rotation of atoms or atomic groups around the single bonds that make up the polymer chain, are able to take on a variety of conformations. The action of mechanical or other external fields can also change the conformation of macromolecules.

The conformation of a macromolecule is the spatial arrangement of atoms and groups of atoms, which is determined by the set and sequence of configurational isomers and their relative mutual arrangement in the chain, due to thermal motion or external influences on the macromolecule.

As a result of thermal motion or other external influences on a macromolecule, an innumerable number of different conformations are usually realized for each configuration of the polymer chain. The ability to change the conformation of the chain determines the most important property of macromolecules - their flexibility. Let us introduce some ideas about the size of the polymer chain.

An isolated macromolecule chain in the process of thermal motion can take on a large number of different conformations, therefore the dimensions of the chain are characterized by the average distance between its ends (in this case, the rms value of the distance is usually used). The concept of the rms value of the radius of gyration of the chain is also used. The value is the average square of the distance (ri) of all mass elements of the chain from its center of inertia.

For macromolecules of linear polymers, the square of the average radius of gyration is usually 6 times smaller than the square of the average distance between the ends of the chain, i.e.

Consider the relationship between the sizes of macromolecules and the main parameters of the chain: the length of the bonds included in it (l), their number (N), bond angles () for various models.